1. Field of the Invention
The invention concerns an electroluminescent device (hereinafter referred to as an EL device) used, for example, in light emitting-type segment and matrix displays of meters or in displays of various types of computer terminals.
2. Description of Related Art
In the past, EL devices that utilize the phenomenon of light emission when electric fields are applied to fluorescent materials like zinc sulfide (ZnS) have been widely noted as a component for light emitting-type, flat panel displays. FIG. 3 is a diagram that shows a cross-sectional structure of a conventional EL device 10. The EL device 10 is formed by sequentially laminating on top of an insulating glass base 1 a first electrode 2 made up of an optically transparent ITO film, a primary insulating layer 3 made up of tantalum pentoxide (Ta.sub.2 O.sub.5) or the like, a light emitting layer 4, a second insulating layer 5 and a second electrode layer 6 made up of ITO film.
The ITO films 2,6 are transparent conductive films made by doping indium oxide (In.sub.2 O.sub.3) with tin (Sn). This composition has been widely used as in the past as a transparent electrode because of its low electrical resistance. The light from light emitting layer 4 varies with, for example, the type of additive added to the zinc sulfide (ZnS) used therein. For example, a yellowish-green colored light is emitted if terbium (Tb) is added as a light emitting center, a reddish-orange light when samarium (Sm) is added, a bluish light when thulium (Tm) is added and a bluish-green colored light is emitted if cerium (Ce) is added to strontium sulfide (SrS) as the light emitting center.
For the EL device 10 having the above structure, the use of zinc sulfide (ZnS) or zinc selenide (ZnSe) added to terbium (Tb), or equivalently, other sulfur or selenium compounds as the component material of a light emitting layer 4 for increasing the brightness of light emissions using electronic beam (EB) deposition, sputtering or the like has been investigated. In recent times, investigations using CVD (Chemical Vapor Deposition) have also been performed.
Examples of the CVD method include those based on halogen element gas transportation and those that vaporize halide compounds at high temperatures and transport them to the reaction chamber or the like. However, for the methods described above, because of high vaporization temperatures, difficulties in controlling vapor amounts and likely changes in the ratio of II and VIB-group gases supplied to the reaction chamber with the material of the light emitting center, light emitting layers of high quality cannot be developed consistently.
The Metalorganic Chemical Vapor Deposition (MOCVD) method that uses organic metals, e.g. dipivaloylmethanate compounds, has been gaining notice as a solution to this problem. In general, the MOCVD method follows a methodology wherein an organic metal in liquid form is bubbled with hydrogen (H.sub.2) to contain the organic metal inside the hydrogen and then supplied to the reaction chamber. However, rare earth elements and organic metals of the alkaline earth metal elements are generally fine solid particles at normal temperatures and have melting points of 150.degree. C. and above. Accordingly, the supply method for the source gas material employs a technique wherein a source material container filled with rare earth elements and organic metals of the alkaline earth metal elements is first heated in a heating oven, after which a transport gas (carrier gas), e.g., H.sub.2 or the like is introduced and then vaporized organic metal contained inside the hydrogen (H.sub.2) is transported to the reaction chamber.
However, if the organic metal is solid, then decomposition or the like of the source material due to the heat occurs and thus, large amounts of source material gas cannot be obtained because of sublimation. Accordingly, as was noted in the 1991 MRS Symposium Proceedings Vol. 222 pp. 315-322, when a solid organic metal is used as a source material during the formation of the host substance of the light emitting layer, it has been reported that if the temperature applied to the source material surpasses a certain level, the source material begins to decompose, its characteristics change and thus, the speed of the formation of the light emitting layer is said to vary widely. In addition, the speed of the formation of the light emitting layer is a problem because, as calculated in the above paper, a long period of time unsuitable for production is needed to form a film of the light emitting layer which has a practical thickness. Accordingly, even if the source material is heated to derive large amounts of sublimated gas of the organic metal that is the source material, only very small amounts of source material can be derived because source materials decompose even before a practical amount of gas can be obtained, and thus, achieving practical film formation speed is not possible.